In the drilling industry, seals are used in various applications including rotating blowout preventers, swab cups, pipe and Kelly wipers, sucker rod guides, tubing protectors, stuffing box rubbers, stripper rubbers for coiled tubing applications, snubbing stripper rubbers, and stripper rubbers for rotating control heads or diverter/preventers. Stripper rubbers, for example, are utilized in rotating control heads to seal around the rough and irregular outside diameter of a drillstring of a drilling rig.
Stripper rubbers are currently made so that the inside diameter of the stripper rubber is considerably smaller (usually about one inch) than the smallest outside diameter of any component of a drillstring. As the components move longitudinally through the interior of the stripper rubber, a seal is continuously affected.
Stripper rubbers affect self-actuating fluid-tight seals in that, as pressure builds in the annulus of a well, and in the bowl of the rotating control head, the vector forces of that pressure bear against the outside surface or profile of the stripper rubber and compress the stripper rubber against the outside surface of the drillstring. The pressure forces complement the stretch-fit forces already present in the stripper rubber. The result is an active mechanical seal the increases the seal strength as the well bore pressure increases.
Well pressure forces often distort the elastic profile of a stripper rubber, deforming the shape from that of a cone to that of a donut. Lowering an oil tool through the stripper rubber often causes the deformed, rolled up, rubber to temporarily uncurl, but the rubber quickly returns to the deformed donut shape once it is re-pressurized. Wear and tear on the stripper rubber occurs, therefore, not only from frictional forces between the rubber and a longitudinally moving oil tool, but from the mechanical forces acting on the rubber as it rolls up and unrolls during drilling operations.
Stripper rubbers seal around rough and irregular surfaces of varying diameters such as those found around a drill pipe, tool joints, or a Kelly, and are operated under drilling conditions where strength and resistance to wear are prized attributes. When using a stripper rubber in a rotating control head, the longitudinal location of the rotating control head is fixed due to the mounting of a stripper rubber onto a bearing assembly. The bearing assembly allows the stripper rubber, or stripper rubbers, to rotate with the Kelly or drillstring, but restrains the stripper rubber from longitudinal, axial, movement. Wear of the interior surface of a stripper rubber is caused by relative longitudinal movement of the drillstring components, including the end to end coupling areas of larger diameter joints and the larger diameter of tools that bear against a stripper rubber.
Wear and tear upon a stripper rubber from frictional and mechanical forces will, over a period of time, cause a thinning or weakening of the elastic material to the point that the stripper rubber will fail. Such wear is exacerbated by the movement of multiple lengths of a drillstring through the stripper rubber, such as when a drillstring is “tripped” into or out of the well. Furthermore, the stretch-fit of the rubber rapidly becomes exhausted, and the rubber fails to seal the well. Rapid exhaustion of the rubber remains a persistent problem, and requires frequent, sometimes as often as weekly, replacement of the stripper rubber.
Metal structures, called “inserts,” embedded in the rubber portion of a stripper rubber are used to provide connectors and structural support to the rubber. For example, U.S. Pat. No. 5,647,444, issued to the present inventor, discloses a dual stripper rubber apparatus. Each stripper rubber provides a pair of circular spring inserts that circumnavigate the packer perpendicular to the drillstring bore. The springs provide structural support to the rubber, yet permit the rubber to dilate so that pipe joints or tools can pass through the drillstring bore of the stripper rubbers.
Wear is present in all drilling and production applications where a rubber seal or wiper is subjected to the relative movement of a component such as a drillstring tool. A long-felt need persists for a rubber seal or wiper that is resistant to wear, will withstand the great bore hole pressures of modern wells, and is capable of a longer service life than has been heretofore possible.